This invention relates generally to magnetic sensors for detecting external magnetic fields using a ferromagnetic free layer, and in particular to magnetic sensors such as spin valves or tunnel valves in which the free layer is exchanged coupled with an antiferromagnetic layer.
Thin film magnetoresistive heads have been used in magnetic data storage devices for several years. The fundamental principles of magnetoresistance including anisotropic magnetoresistance (AMR), giant magnetoresistance (GMR) and spin tunneling have been well-known in the art for some time. Magnetic read heads, e.g., those used in the field of magnetic recording, use magnetic sensors built on these principles and other effects to produce devices capable of reading high density magnetically recorded data. In particular, three general types of magnetic read heads or magnetic readback sensors have been developed: the anisotropic magnetoresistive (AMR) sensor, the giant magnetoresistive (GMR) sensor or GMR spin valve and tunnel valve sensor. The construction of these sensors is discussed in the literature, e.g., in U.S. Pat. No. 5,159,513 or U.S. Pat. No. 5,206,590.
Magnetoresistive sensors rely on a ferromagnetic free layer to detect an external magnetic field, e.g., the field produced by data stored in the form of magnetic domains in a magnetic storage medium. The free layer typically has a low coercivity and low anisotropy and thus an easily movable or rotatable magnetic moment which responds to the external field. The rotation of the free layer""s magnetic moment causes a change in the resistance of the device by a certain value xcex94R (measured between electrical contacts). (In general, the larger the value of xcex94R in relationship to total resistance R, i.e., the larger xcex94R/R the better the sensor.) This change in resistance due to rotation of the magnetization of the free layer can thus be electronically sensed and used in practical applications such as reading of magnetic data.
An important concern in the design of the sensor is the longitudinal bias of the free layer. In particular, the free layer must be biased by a hard bias so that it is essentially in a single domain state. Deviations from a single domain state are mostly due to edge effects and corners and demagnetizing field effects as the sensor is excited by the external magnetic field. Also, the free layer has to be properly biased in the quiescent state to ensure a linear or essentially linear response with maximum dynamic range. When the free layer is allowed to have more than one magnetic domain, then the free layer experiences Barkhausen jumps and other domain reorientation phenomena, as is known in the art. This is highly undesirable as it produces noise and worsens the signal-to-noise ratio (SNR) of the sensor.
In order to provide the biasing field and prevent noise some of the prior art sensors deploy a longitudinal biasing scheme or a hard bias layer having a high coercivity. Typically, such scheme uses a magnetic material placed essentially in the same plane as the free layer next to and close to it. For example, a hard bias material such as CoPt hard magnet alloy can be used in the form of a hard bias tab. This biasing scheme ensures that the free layer has a single magnetic domain. For more details on longitudinal biasing the reader is referred to U.S. Pat. No. 5,729,410 to Fontana, Jr. et al.
To properly bias the free layer other prior art solutions employed in spin valve and tunnel valve sensors balance the forces of the magnetostatic field Hm set up by the pinned layer, the interlayer coupling field Hi between the free layer and the pinned layer (due to Nexc3xa9l orange peel, pinholes, oscillatory coupling etc.) and the current-induced field Hj caused by current flowing through the sensor structure. This approach is illustrated in FIG. 1 in a typical spin valve 12 with a seed layer 14 on one side and a cap layer 24 on the other side. Sandwiched between layers 14, 24 are a ferromagnetic free layer 16, a spacer layer 18, and a pinned layer 20 which is exchange-coupled with an antiferromagnetic layer 22. The arrows indicate the overall magnetizations of layers 16, 20 and 22. A current j flowing through spin valve 12 between electrical contacts (not shown) is indicated by an arrow. For optimal performance free layer 16 has to be properly magnetically biased so that its response to an external magnetic field, e.g., a field created by a magnetic recording medium, is highly linear and so that there is maximum dynamic range (i.e., so that the responses to a positive and negative signals are both as large as possible before there is signal saturation). This is accomplished by maintaining the magnetization of free layer 16 substantially at 90xc2x0 to the magnetization of pinned layer 20 in the absence of a signal or external magnetic field. Thus, the forces of fields Hm, Hi and Hj as well as any other forces (e.g., due to uniaxial anisotropy, shape anisotropy, etc.) acting on free layer 16 have to be balanced such that the transverse component of the sum of the forces acting on the free layer cancel:
Hi+Hj+Hm=0xe2x80x83xe2x80x83(Eq. 1)
Thus the transverse components of these vectors add to zero. In practice, these vectors are aligned along a transverse direction as shown and that is why vector addition can be replaced by simple addition. Under ideal conditions equation 1 is satisfied over entire free layer 16 such that free layer 16 experiences zero field and is highly sensitive to the external magnetic field.
The problem with balancing the transverse components of Hi, Hj and Hm is that in a practical device such balance is hard to achieve. Generally, magnetostatic field Hm is spatially non-uniform in free layer 16 with substantial fields of 100-200 Oe present at a bottom surface 26 (typically the air-bearing surface) and at a top surface 28, and substantially lower fields in the interior of free layer 16. The result is a spatially non-uniform orientation of the magnetization in free layer 16. Field Hi is uniform across sensor 12 but is not easily controlled over a wide range and can not be always made small. Also, Hi and Hm depend on a height of free layer 16 or the stripe height between bottom surface 26 and top surface 28. This height can not be easily controlled in practice. Field Hj is nearly uniform except for variations caused by current bunching near the leads.
Thus, equation 1 is typically constraining since the values of Hm, Hi, and Hj cannot be independently optimized, especially if large magnetoresistance is to be obtained because the optimization of magnetoresistance often requires layer thicknesses incompatible with the constraint of equation 1. The result is a non-optimal compromise.
In particular, it would be desirable to make Hm as small as possible so that the Hm-related nonuniformities are minimized. It would also be desirable to make Hj relatively large to be able to use a large bias current for increased sensitivity. To satisfy the equation, then, Hi must be made relatively large to help balance Hj. This poses problems because Hi is sensitively dependent upon the surface textures of the layers. It is difficult to fabricate the layers so that a large, well-defined value of Hi is provided consistently. Therefore, there exists a practical limit on the magnitude of Hi.
Consequently, Hm and Hj cannot have vastly different magnitudes. At best, state of the art GMR sensors compromise between the competing benefits of low Hm values, high Hj values and low Hi values.
There thus exists a need for developing a proper scheme for longitudinal biasing of the free layer of a magnetic sensor. In particular, there exists a need for balancing fields Hm, Hi and Hj acting on the free layer without sacrificing the ability to optimize the values of these fields for good sensor performance. More precisely, there exists a need for magnetic sensors such as spin valve or tunnel valve sensors that have very low Hm and controlled Hi, yet allow high values of Hj while keeping the free layer properly biased.
Accordingly, it is a primary object of the present invention to provide a structure for longitudinal biasing of a free layer in a magnetic sensor, such as a spin valve or a tunnel valve. The improved bias is provided while allowing for low values of magnetostatic field Hm, low values of interlayer coupling field Hi and high values of field Hj due to current flow through the sensor.
It is another object of the invention to ensure that the bias of the free layer is more spatially uniform and that the sensitivity of the biasing has a reduced sensitivity to stripe height variation.
Yet another object of the invention is to provide a transverse bias to the free layer while maintaining reduced free layer stiffness.
These and other objects and advantages will be apparent upon reading the following description and reviewing the accompanying drawings.
These objects and advantages are attained by a magnetic sensor which detects an external magnetic field with the aid of a ferromagnetic free layer having a magnetic moment responsive to the external magnetic field. The magnetic sensor has a first antiferromagnetic layer which is magnetically exchange-coupled to the free layer to produce an exchange bias field He. This exchange bias field He acts on the free layer to bias its magnetic moment along a certain orientation. In this manner, the invention provides an additional field, the exchange bias field He, which gives the designer an additional degree of freedom in balancing a total transverse internal magnetic field Ht which is due to other fields generated by the sensor itself. For example, in a sensor such as a spin valve fields Hm, Hi, Hj and potentially other fields act on the free layer to produce total transverse internal magnetic field Ht which requires balancing.
The value of exchange bias field He is set by selecting a certain thickness and a certain composition of the antiferromagnetic layer. The magnetic sensor of the invention can also have a first non-magnetic spacer layer interposed between the free layer and the first antiferromagnetic layer. Such spacer layer can have a thickness in the range of 0.1 to 3.0 nanometers. Alternatively, the first antiferromagnetic layer can be in contact with the free layer.
The sensor can be a spin valve sensor or a tunnel valve sensor. In general, this principle can be applied in any magnetic sensor using the free layer and the magnetoresistive effect to detect the external magnetic field.
The sensor of the invention can be used in a magnetic read head, e.g., a magnetic read head for reading data recorded in a magnetic recording medium in the longitudinal recording mode. In this application the orientation in which the free layer""s magnetic moment is biased should be longitudinal. Of course, the sensor can also be used in other magnetic recording schemes.
There are numerous types of sensors in which the antiferromagnetic layer exchange-coupled to the free layer can be used. Preferably, these sensors use a pinned layer having a magnetic moment further fixed or stabilized by an antiferromagnetic layer. In other words, these sensors use an anti-parallel pinned layer. A magnetostatic field Hm produced by the anti-parallel pinned layer is advantageously small and can be made near zero so that it is easy to balance.
The first antiferromagnetic layer which is exchange-coupled to the free layer can be made of various materials. Preferably, materials containing Mn are used. Suitable alloys containing Mn include FeMn, PtMn, IrMn, PdPtMn and NiMn. Preferably, the first antiferromagnetic layer has a thickness between 20 and 400 xc3x85.
A preferred structure of a magnetic sensor for use in magnetic read heads uses an anti-parallel (AP) pinned trilayer with a second antiferromagnetic layer for anti-parallel pinning the AP pinned trilayer. The entire sensor is thus constructed of the following layers: a second antiferromagnetic layer; an AP pinned trilayer including a first ferromagnetic layer comprising Co, an AP spacer layer comprising Ru, and a second ferromagnetic layer comprising Co; a second non-magnetic spacer layer comprising Cu; a nanolayer comprising Co; a ferromagnetic free layer comprising NiFe; and a first antiferromagnetic layer exchange-coupled to the ferromagnetic free layer.